In a magnetic switching using spin transfer torque (hereinafter referred to as “STT”), writing current become small as the element size decreases, so that it is suitable for high density and low power memory element. Recently, magnetic random access memory (MRAM) utilizing such magnetic switching of STT (STT-MRAM) is attracting much attention.
STT-MRAM is composed of a magnetic tunnel junction (MTJ) element (hereinafter sometimes referred to as “MTJ element”). The MTJ element adopts a structure where a tunnel barrier layer (tunnel insulating film) is sandwiched between a reference (fixed) layer having a fixed magnetization direction and a recording (free) layer in which the magnetization direction varies.
Performance of the MTJ element is represented by tunnel magnetoresistance ratio (TMR ratio), thermal stability and threshold current. The TMR ratio is a value defined by (Rap−Rp)/Rp (wherein Rp represents a resistance value in a state where magnetization of the reference layer and magnetization of the recording layer that are arranged adjacent to the barrier layer are arranged in parallel, and Rap represents a resistance value in a state where magnetization of the reference layer and the magnetization of the recording layer that are arranged adjacent to the barrier layer are arranged in antiparallel). Further, thermal stability is a value that is proportional to Keff−V/kBT (wherein Keff represents effective magnetic anisotropy energy density of the recording layer, V represents volume of the recording layer, kB represents Boltzmann's constant, and T represents absolute temperature). Generally, the TMR ratio of the MTJ element should preferably be greater, and the value having divided the thermal stability by threshold current should preferably be greater.
If the MTJ element has a magnetic anisotropy with perpendicular easy axis, the magnetic switching path is the same between STT switching and thermal switching. Meanwhile, if the MTJ element has an in-plane magnetic anisotropy, magnetic switching path caused by STT differs from that caused by thermal inversion. In this case, inversion by STT causes magnetization to flow through a plane-perpendicular direction having a large demagnetizing field, whereas in thermal inversion, magnetization flows through an in-plane direction having a small demagnetizing field. As a result, in in-plane magnetization, ratio of thermal stability to threshold current is small compared to perpendicular magnetization. Therefore, perpendicular magnetization-type MTJ element is attracting much attention recently, and such perpendicular magnetization-type MTJ element is being used.
As an example of such perpendicular magnetization-type MTJ element, an element having a high TMR ratio, high thermal stability and low threshold current is developed by using a ferromagnetic layer formed of CoFeB and an MgO insulation film (refer to Japanese Patent Literature 1), and the use of such materials as bases is investigated.
Further, in order to improve perpendicular magnetic anisotropy, a structure (double interface structure) such as an MgO (barrier layer)/CoFeB (recording layer)/MgO (protective layer) structure in which the recording layer (CoFeB) is sandwiched between the barrier layer (MgO) and the protective layer (MgO) containing oxygen has been developed (refer for example to Japanese Patent Literature 1). Further, materials adopting a double interface structure in which a conductive oxide layer is used as the protective layer or further having a metal cap layer arranged above the protective layer are developed (refer for example to Japanese Patent Literature 2).
The MTJ element having such double interface structure can realize a recording layer thickness that is greater than that of a tunnel junction element that does not have an MgO protective layer, due to two perpendicular magnetic anisotropies that are caused in the CoFeB/MgO interface under the recording layer and above the recording layer. Since thermal stability increases in proportion to the recording layer thickness, thermal stability can be improved by increasing the film thickness. Simultaneously, by increasing the recording layer thickness, damping constant α of the recording layer may also be reduced. Since the write current value is in proportion to the damping constant α, the write current value can be reduced at the same time. As a result, MTJ element having a double interface structure has high thermal stability and small write current, that is, the value obtained by dividing thermal stability by threshold current is high.
Meanwhile, in the MTJ element that has no MgO protective layer, a protective layer of Ta and the like is formed instead of the MgO protective layer on the recording layer. In this case, since Ta absorbs boron through heat processing, CoFeB is crystallized and a high TMR ratio is obtained.
However, since the MTJ element having a double interface structure described in Patent Literatures 1 and 2 sandwiches CoFeB with MgO, and it does not have a cap formed of Ta and the like, diffusion of boron by heat processing does not easily occur. Therefore, there was a drawback in that CoFeB is not crystallized by annealing and TMR ratio is deteriorated.
In order to prevent such deterioration of TMR ratio, an MTJ element having a thin nonmagnetic layer such as Ta inserted between the recording layer is proposed (refer for example to Non-Patent Literature 1). In this MTJ element, the nonmagnetic layer formed of Ta and the like absorbs boron through heat processing and CoFeB is crystallized, so that a high TMR ratio is obtained.
Further, an MTJ in which a reference layer of CoFeB interposing a thin Ta of approximately 4 Å so-called a magnetic coupling layer is formed on a film having a high perpendicular magnetic anisotropy such as a [Co/Pt] multilayer film serving as the magnetization fixed layer is proposed (refer for example to Non-Patent Literature 1). In a CoFeB/MgO/CoFeB tunnel junction, a high TMR ratio is obtained when the CoFeB and the MgO adopt a bcc crystal orientation. Since the [Co/Pt] has a crystal alignment property of fcc, if the CoFeB reference layer is directly formed thereon, a high TMR ratio cannot be obtained since CoFeB is crystally aligned to the fcc crystal orientation of [Co/Pt]. The magnetic coupling layer is used to disconnect the [Co/Pt] crystal alignment at this layer, so as to align the CoFeB in the bcc direction and to achieve a high TMR ratio.